Heterogenically catalysed gas-phase partial oxidation method for precursor compounds of (meth)acrylic acid

ABSTRACT

The invention relates to a method for the heterogenically catalysed gas-phase partial oxidation of precursor compounds of (meth)acrylic acid in a fixed catalyst bed, containing as the catalyst an activated mass of mixed oxide, shaped to form a geometric body. Said geometric body is a geometric base body, into whose surface a cavity is incorporated.

[0001] The present invention relates to a process for theheterogeneously catalyzed gas-phase partial oxidation of a precursorcompound of (meth)acrylic acid to (meth)acrolein and/or (meth)acrylicacid by passing a reaction gas starting mixture comprising the precursorcompound, molecular oxygen and, if required, a gas which is inert withrespect to the catalytic gas-phase partial oxidation, at elevatedtemperatures, through a fixed catalyst bed which contains, as thecatalyst, a mixed oxide active material shaped into a geometric body,this geometric body being a geometric base body into whose surface atleast one cavity has been introduced.

[0002] In this publication, (meth)acrylic acid is used as abbreviatednotation for methacrylic acid or acrylic acid. In this publication,(meth)acrolein is used as abbreviated notation for methacrolein oracrolein.

[0003] (Neth)acrylic acid, either as such or in the form of its esters,is important in particular for the preparation of polymers for a verywide range of applications, for example for use as adhesives.

[0004] In this publication, precursor compounds of (meth)acrylic acidare understood very generally as meaning organic compounds from which(meth)acrylic acid is obtainable by heterogeneously catalyzed gas-phasepartial oxidation. They are usually alkanes, alkanols, alkenes oralkenals which contain 3 or 4 carbon atoms. (Meth)acrylic acid isparticularly advantageously obtainable, for example, by heterogeneouslycatalyzed gas-phase partial oxidation of propane, propene, tert-butanol,isobutene, isobutane, isobutyraldehyde or (meth)acrolein. However, otherpossible precursor compounds are those from which the actualC₃-/C₄-precursor compound is only formed as an intermediate during theheterogeneously catalyzed gas-phase partial oxidation. An example is themethyl ether of tert-butanol.

[0005] In the heterogeneously catalyzed gas-phase partial oxidation, theprecursor compounds described above are passed, as starting gases, as arule diluted with inert gases such as molecular oxygen, CO, CO₂, inerthydrocarbons and/or steam, as a mixture with molecular oxygen, atelevated temperatures (usually from about 200 to 450° C.) and, ifrequired, superatmospheric pressure, over transition metal mixed oxideactive materials (e.g. containing Mo, Cu and P, or Mo, Bi and Fe, or Mo,V and W, or Mo, V, Te and Nb (where P is not mentioned, it is as a rulenot present)) and are converted by oxidation either directly into(meth)acrylic acid or, in a first step, into its precursor compound(meth)acrolein (cf. for example DE-A 4 405 059, EP-A 253 409, EP-A92097, DE-A 4 431 957, DE-A 4 431 949, CN-A 1 105 352, WO 97/36849, EP-A608 838, EP-A 714 700, EP-A 700 893, EP-A 700 714, DE-A 19 815 279, DE-A10 046 672 and DE-A 10 034 825).

[0006] The mixed oxide active materials are shaped into moldings havinga very wide range of geometries and the moldings are combined to give afixed bed through which the reaction gas starting mixture containing theprecursor compound is passed at elevated temperatures. During contactwith the mixed oxide active material, the desired partial oxidationtakes place. If required, the catalyst moldings may also be diluted withinert moldings.

[0007] The shaping of the mixed oxide active material can be effected,for example, by compacting mixed oxide active material in powder form togive the desired catalyst geometry (for example by tabletting orextrusion). The resulting catalysts are referred to as unsupportedcatalysts. For the production of unsupported catalysts, if necessaryassistants, e.g. graphite or stearic acid as lubricants and/or moldingassistants and reinforcing agents, such as microfibers of glass,asbestos, silicon carbide or potassium titanate, may be present inaddition to the mixed oxide catalyst active material in powder form.

[0008] Of course, the shaping can also be effected, for example, byapplying mixed oxide active material in powder form to preshaped inertor active catalyst supports of suitable geometry. Coated catalysts areobtained thereby.

[0009] The shaping methods described can also be used starting fromprecursor materials of the mixed oxide active materials. The conversioninto the active catalysts is effected as a rule afterward by thermaltreatment at elevated temperatures.

[0010] Finally, the supported catalysts in which the metal oxide activematerial is absorbed into the pores of inert supports and/or producedtherein may also be mentioned. Detailed descriptions of processes forthe production of catalyst moldings of mixed oxide catalyst materialssuitable according to the invention are to be found, for example, inEP-A 700 893, DE-A 10 063 162, DE-A 10 046 957, DE-A 19 948 523, EP-A700 714, EP-A 417 723, DE-A 3300044, EP-A 552 287, EP-A 714 700, DE-A 10059 713 and DE-A 10 051 419.

[0011] The catalyst moldings, either as such or as a mixture with inertmoldings (for example the inert supports which can be used for theproduction of coated catalysts), can be converted into fixed catalystbeds. These fixed catalyst beds may be present, for example, in thetubes of tube-bundle reactors (cf. for example EP-A 700 893 and EP-A 700714) or on the trays of tray reactors.

[0012] Spheres and cylinders are recommended as typical geometries forthe unsupported catalyst, coated catalyst and supported catalystmoldings in the relevant prior art for the relevant gas-phase partialoxidations.

[0013] Very recently the use of catalyst moldings whose geometric bodyis a ring, i.e. a cylinder (as geometric base body) having a tubeintroduced into its surface as a cavity was also recommended for theprocess for the heterogeneously catalyzed gas-phase partial oxidation ofprecursor compounds of (meth)acrylic acid to (meth)acrolein and/or(meth)acrylic acid (cf. for example DE-A 19 948 523, DE-A 10 063 162,EP-A 184 790, DE-C 3300044 and EP-A 714 700).

[0014] In a similar manner, EP-A 417 723 and EP-A 355 664 also recommendthe use of geometric catalyst bodies which correspond to a geometricbase body into whose surface at least one cavity has been introduced.For example, cylinders, cubes or prisms are considered as possiblegeometric base bodies.

[0015] However, the disadvantage of the abovementioned geometriccatalyst bodies which are recommended in the prior art for the processfor the heterogeneously catalyzed gas-phase partial oxidation ofprecursor compounds of (meth)acrylic acid to (meth)acrolein and/or(meth)acrylic acid and which differ from spheres and cylinders is that,in all cases, either the ratio of the volume of the geometric catalystbody (V_(B)) to the volume of the geometric base body (V_(BA)), i.e.V_(B):V_(BA), is >0.6 and/or the ratio of the external surface area ofthe geometric catalyst body (A_(B)) to V_(B), i.e. A_(B):V_(B), is <22cm⁻¹.

[0016] This is disadvantageous in that the selectivity of the formationof the desired products achieved using such geometric catalyst bodies isnot completely satisfactory.

[0017] It is an object of the present invention to provide a process forthe heterogeneously catalyzed gas-phase partial oxidation of a precursorcompound of (meth)acrylic acid to (meth)acrolein and/or (meth)acrylicacid by passing a reaction gas starting mixture comprising the precursorcompound, molecular oxygen and, if required, a gas which is inert withrespect to the catalytic gas-phase partial oxidation, at elevatedtemperatures, through a fixed catalyst bed which contains, as thecatalyst, a mixed oxide active material shaped into a geometric body,this geometric body being a geometric base body into whose surface atleast one cavity has been introduced, which process ensures improvedselectivity with respect to the formation of the desired products.

[0018] We have found that this object is achieved by a process for theheterogeneously catalyzed gas-phase partial oxidation of a precursorcompound of (meth)acrylic acid to (meth)acrolein and/or (meth)acrylicacid by passing a reaction gas starting mixture comprising the precursorcompound, molecular oxygen and, if required, a gas which is inert withrespect to the catalytic gas-phase partial oxidation, at elevatedtemperatures, through a fixed catalyst bed which contains, as thecatalyst, a mixed oxide active material shaped into a geometric body,this geometric body being a geometric base body into whose surface atleast one cavity has been introduced, wherein the ratio of the volume ofthe geometric body V_(B) to the volume of the geometric base bodyV_(BA)≦0.63 and the ratio of the external surface area of the geometricbody A_(B) to V_(B)≧22 cm⁻¹.

[0019] According to the invention, A_(B) to V_(B) may thus be ≧23 cm⁻¹or ≧24 cm⁻¹ or ≧25 cm⁻¹ or ≧26 cm⁻¹ or ≧27 cm⁻¹.

[0020] In the novel process, the ratio of A_(B) to V_(B) is as a rule≦30 cm⁻¹.

[0021] Furthermore, the ratio V_(B):V_(BA) may be, according to theinvention, ≦0.62 or ≦0.61 or ≦0.60 or ≦0.58 or ≦0.56 or ≦0.54 or ≦0.52or ≦0.50 or ≦0.48 or ≦0.45. In the novel process, as a rule V_(B):V_(BA)is ≧0.30, frequently ≧0.35 or ≧0.40.

[0022] According to the invention, it is advantageous if A_(B) to V_(B)is very large and V_(B):V_(BA) is very small.

[0023] According to the invention, it is important that V_(B), V_(BA)and A_(B) are those volumes and surface areas which the eye is capableof perceiving visually when viewing the geometric body, i.e. internalvolumes and surface areas which originate from finely divided poresand/or cracks in the material of the geometric body are not included inV_(B), V_(BA) and A_(B).

[0024] In the novel process, preferably at least 25% (of the number),better at least 50%, especially at least 75%, particularly preferably100%, of the totality of the mixed oxide material contained in the fixedcatalyst bed are shaped into geometric bodies for which theabovementioned conditions are fulfilled, i.e. for whichV_(B):V_(BA)≦0.63 and for which A_(B) to V_(B)≧22 cm⁻¹. If the fixedcatalyst bed used for the novel process additionally contains inertmoldings for the purpose of dilution, it is preferable, according to theinvention, if furthermore at least 25% (of the number), better at least50%, especially at least 75%, particularly preferably at least 100%, ofall inert moldings present are geometric bodies for which theabovementioned conditions are fulfilled, i.e. for whichV_(B):V_(BA)≦0.63 and for which A_(B) to V_(B)≧22 cm⁻¹.

[0025] Suitable geometric base bodies for the novel process are allthose which are discussed in EP-A 552 287. These are in particularcylinders, pyramids, cones, cubes, right parallelepipeds, prisms,spheres, truncated cones and truncated pyramids.

[0026] The figures attached to this publication show some novelgeometric bodies which are suitable in principle. These are specificallyas follows:

[0027] FIGS. 1A,B: cylinder as geometric base body; cavities as roundedgrooves running substantially perpendicularly from top to bottom,introduced equidistantly into the surface of the base body; FIG. 1Bshows the view from above.

[0028] FIGS. 2A,B: this figure shows a variation of the geometry shownin FIGS. 1A,B.

[0029] FIGS. 3A,B: this figure shows a variation of the geometry shownin FIGS. 1A,B and additionally contains a central hole.

[0030] FIGS. 4A,B: cylinder as geometric base body; cavity introduced ascentral hole. FIG. 4B shows the view from above.

[0031] FIGS. 5A,B: this figure shows a variation of the geometry shownin FIGS. 1A,B. Angled grooves.

[0032] FIGS. 6A,B: cylinder as geometric base body; cavity as centralhole and continuously wound spiral introduced into the surface of thebase body. FIG. 6B shows the view from above.

[0033] FIGS. 7A,B: cylinder as geometric base body; cavity as a roundedgroove running substantially perpendicularly from top to bottom andconnected to central hole. FIG. 7B shows the view from above.

[0034] FIGS. 8A,B: pyramid having a square base as base body; cavitiesas rounded grooves introduced with equal spacing on the outer surface atthe edges of the pyramid. FIG. 8B shows the view from above.

[0035] FIGS. 9A,B: pyramid having a square base as base body; cavitiesas rounded grooves introduced with equal spacing on the outer surfaceinto the sides of the pyramid and running obliquely from top to bottom.FIG. 9B shows the view from above.

[0036] FIGS. 10A,B: cone having a circular base as base body. Cavitiesas rounded grooves introduced with equal spacing on the outer surface ofthe cone and running obliquely from top to bottom. FIG. 10B shows theview from above.

[0037] FIGS. 11A,B: cube as base body; cavities as angled groovesintroduced with equal spacing on the outer surface of the base body, onthe sides, and running substantially perpendicularly from top to bottom.FIG. 11B shows the view from above.

[0038] FIGS. 12A,B: cube as base body; cavities as angled grooves on theouter surface, introduced into opposite sides and running substantiallyperpendicularly from top to bottom. FIG. 12B shows the view from above.

[0039] FIGS. 13A,B: cube as base body; cavities as rounded troughsintroduced with equal spacing on the outer surface of the base body, onthe sides, the upper surface and the lower surface. FIG. 13B shows theview from above.

[0040] FIGS. 14A,B: sphere as base body; cavities as rounded troughsintroduced with equal spacing on the outer surface. FIG. 14B shows thecross-section at the equator.

[0041] Mixed oxide catalyst materials shaped into rings areadvantageously used as catalysts for the novel process. If they are usedin a form diluted with inert moldings, the inert moldings advantageouslyalso have annular geometry. Preferably, the geometries of inert moldingand of geometric catalyst body are identical.

[0042] According to the invention, the following annular geometries arepreferred (in each case external diameter×height×internal diameter):

5.5 mm×3 mm×3.5 mm (A _(B) :V _(B)=26.7; V _(B) :V _(BA)=0.595);

6 mm×3 mm×4 mm (A _(B) :V _(B)=26.7; V _(B) :V _(BA)=0.556);

7 mm×3 mm×4.5 mm (A _(B) :V _(B)=22.7; V _(B) :V _(BA)=0.587);

7 mm×3 mm×5 mm (A _(B) :V _(B)=26.7; V _(B) :V _(BA)=0.490).

[0043] The end faces of the rings may also be curved, as described inEP-A 184 790, for example in such a way that the radius of curvature ispreferably from 0.4 to 5 times the external diameter. However, allgeometries which are mentioned individually in EP-A 552 287 and forwhich A_(B):V_(B) is ≧22 and V_(B):V_(BA) is ≦0.6 are of course alsosuitable.

[0044] It is furthermore advantageous for the novel process if the emptyvolume of the fixed catalyst bed used (this is the sum of the volumeportions of the fixed catalyst bed which are not occupied by solid whenthe bed is viewed) is ≧50% by volume, based on the total volume of thefixed catalyst bed used (this is the sum of the volume portions of thefixed catalyst bed which are occupied either by solid or by gas whenviewed) and based on 25° C. and 1 atm of the bed.

[0045] According to the invention, the empty volume of the fixedcatalyst bed used may thus be ≧52 or ≧55 or ≧57 or ≧60 or ≧62 or ≧65 or≧67% by volume. In the novel process, the empty volume of the fixedcatalyst bed used is as a rule not more than 70% by volume.

[0046] If the catalyst body used according to the invention for thenovel process has through-holes and if the fixed catalyst bed containingthese catalyst bodies is present in the interior of a tube, it isexpedient according to the invention if the ratio of internal tubediameter to the longest cross-section of a hole is ≦7.5, preferably ≦7,advantageously ≦6.5, frequently ≦6, often ≦5.5. As a rule, this ratio is≧4, generally ≧4.5, often ≧5.

[0047] The novel process is particularly advantageous when the mixedoxide active materials used are such that, in a single pass through thefixed catalyst bed in the novel process, a conversion of the precursorcompound of (meth)acrylic acid of ≧90 mol % results, the selectivity ofthe resulting formation of the desired products (meth)acrolein and/or(meth)acrylic acid frequently being at least 50, preferably at least 75,particularly preferably at least 85, mol %.

[0048] The novel process is particularly suitable for the followingheterogeneously catalyzed gas-phase partial oxidations of precursorcompounds of (meth)acrylic acid (to be carried out in each case in anoxidation stage):

[0049] a) propene to acrolein;

[0050] b) propene to acrylic acid;

[0051] c) acrolein to acrylic acid;

[0052] d) propane to acrolein;

[0053] e) propane to acrylic acid;

[0054] f) isobutene to methacrolein;

[0055] g) isobutene to methacrylic acid;

[0056] h) methacrolein to methacrylic acid;

[0057] i) isobutane to methacrolein;

[0058] j) isobutane to methacrylic acid.

[0059] The mixed oxide active materials required as catalysts for theseheterogeneously catalyzed gas-phase oxidations and the methods forshaping them into geometric bodies suitable according to the inventionare described in the prior art, for example that cited in thispublication.

[0060] A large number of the mixed oxide active materials suitable forthe heterogeneously catalyzed gas-phase partial oxidation of propene toacrolein can be subsumed under the formula I

Mo₁₂Bi_(a)Fe_(b)X¹ _(c)X² _(d)X³ _(e)X⁴ _(f)O_(n)   (I)

[0061] where

[0062] X¹ is nickel and/or cobalt,

[0063] X² is thallium, an alkali metal and/or an alkaline earth metal,

[0064] X³ is zinc, phosphorus, arsenic, boron, antimony, tin, cerium,lead and/or tungsten,

[0065] X⁴ is silicon, aluminum, titanium and/or zirconium,

[0066] a is from 0.5 to 5,

[0067] b is from 0.01 to 5, preferably from 2 to 4,

[0068] c is from 0 to 10, preferably from 3 to 10,

[0069] d is from 0 to 2, preferably from 0.02 to 2,

[0070] e is from 0 to 8, preferably from 0 to 5,

[0071] f is from 0 to 10 and

[0072] n is a number which is determined by the valency and frequency ofthe elements other than oxygen in I.

[0073] They are obtainable in a manner known per se (cf. for exampleDE-A 4023239) and can be used, according to the invention, for exampleeither as such in the shape of rings or in the form of annular coatedcatalyst, i.e. inert supports preshaped into rings and coated with themixed oxide active material.

[0074] In principle, suitable mixed oxide active materials I can beprepared in a simple manner by producing, from suitable sources of theirelemental constituents, a very intimate, preferably finely divided dryblend having a composition corresponding to their stoichiometry andcalcining said dry blend at from 350 to 650° C. The calcination can becarried out either under inert gas or under an oxidizing atmosphere,e.g. air (mixture of inert gas and oxygen) or under a reducingatmosphere (for example a mixture of inert gas, NH₃, CO and/or H₂). Theduration of calcination may be from a few minutes to a few hours andusually decreases with increasing temperature. Suitable sources of theelemental constituents of the mixed oxide active materials I are thosecompounds which are already oxides and/or those compounds which can beconverted into oxides by heating, at least in the presence of oxygen.

[0075] In addition to the oxides, such suitable starting compounds arein particular halides, nitrates, formates, oxalates, citrates, acetates,carbonates, amine complexes, ammonium salts and/or hydroxides(compounds, such as NH₄OH, (NH₄)₂CO₃, NH₄NO₃, NH₄CHO₂, CH₃COOH,NH₄CH₃CO₂ and/or ammonium oxalate, which decompose and/or can bedecomposed at the latest during the subsequent calcination intocompounds escaping completely in gaseous form may additionally beincorporated into the intimate dry blend).

[0076] The thorough mixing of the starting compounds for the preparationof mixed oxide active materials I can be effected in dry or in wet form.If it is effected in dry form, the starting compounds are expedientlyused in the form of finely divided powders and, after mixing and, ifrequired, compaction, are subjected to the calcination. However, thethorough mixing is preferably effected in wet form. Usually, thestarting compounds are mixed with one another in the form of an aqueoussolution and/or suspension. Particularly intimate dry blends areobtained in the mixing process described when the starting materialsused are exclusively sources of the elemental constituents present indissolved form. A preferably used solvent is water. The aqueous materialobtained is then dried, the drying process preferably being carried outby spray-drying of the aqueous mixture at outlet temperatures of from100 to 150° C.

[0077] The mixed oxide active materials of the formula I can be used forthe novel process, for example, in the form of an annular catalystgeometry, where the shaping may be effected before or after the finalcalcination. For example, annular unsupported catalysts can be producedfrom the powder form of the active material or its uncalcined and/orpartially calcined precursor material by compaction to give the desiredcatalyst geometry (for example by extrusion), and, if required,assistants, e.g. graphite or stearic acid as lubricants and/or moldingassistants and reinforcing agents such as microfibers of glass,asbestos, silicon carbide or potassium titanate, may be added.

[0078] The shaping of the pulverulent mixed oxide active material or itspulverulent, still uncalcined and/or partially calcined precursormaterial can of course also be carried out by application to inertcatalyst supports preshaped into annular form. The coating of theannular supports for the production of the coated catalysts is carriedout as a rule in a suitable rotatable container, as disclosed, forexample, in DE-A 2909671, EP-A 293859 or EP-A 714700. For coating theannular supports the powder material to be applied on the support isexpediently moistened and, after application, is dried, for example bymeans of hot air. The coat thickness of the powder material applied tothe annular supports is expediently chosen to be in the range from 10 to1000 μm, preferably from 50 to 500 μm, particularly preferably from 150to 250 μm.

[0079] The support materials used may be conventional porous ornonporous aluminas, silica, thorium dioxide, zirconium dioxide, siliconcarbide or silicates, such as magnesium silicate or aluminum silicate.Supports having pronounced surface roughness are preferred. The use ofsubstantially nonporous, annular steatite supports having a roughsurface is suitable. The fineness of the catalytically active oxidematerials to be applied to the surface of the support is of courseadapted to the desired coat thickness (cf. EP-A 714 700).

[0080] Alternatively, for the purpose of shaping, the annular supportcan also be impregnated with a solution and/or suspension containing thestarting compounds of the elemental constituents of the relevant mixedoxide active material, dried and finally, as described, calcined to givesupported catalysts.

[0081] Advantageous mixed oxide active materials to be used according tothe invention for a novel partial oxidation of propene to acrolein arefurthermore materials of the formula II

[Y¹ _(a′)Y² _(b′)O_(x′)]_(p)[Y³ _(c′)Y⁴ _(d′)Y⁵ _(e′)Y⁶ _(f′)Y⁷ _(g′)Y²_(h′)O_(y′)]_(q)   (II),

[0082] where

[0083] Y¹ is bismuth, tellurium, antimony, tin and/or copper,

[0084] Y² is molybdenum and/or tungsten,

[0085] Y³ is an alkali metal, thallium and/or samarium,

[0086] Y⁴ is an alkaline earth metal, nickel, cobalt, copper, manganese,zinc, tin, cadmium and/or mercury,

[0087] Y⁵ is iron, chromium, cerium and/or vanadium,

[0088] Y⁶ is phosphorus, arsenic, boron and/or antimony,

[0089] Y⁷ is a rare earth metal, titanium, zirconium, niobium, tantalum,rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium,silicon, germanium, lead, thorium and/or uranium,

[0090] a′ is from 0.01 to 8,

[0091] b′ is from 0.1 to 30,

[0092] c′ is from 0 to 4,

[0093] d′ is from 0 to 20,

[0094] e′ is from 0 to 20,

[0095] f′ is from 0 to 6,

[0096] g′ is from 0 to 15,

[0097] h′ is from 8 to 16,

[0098] x′ and y′ are each numbers which are determined by the valencyand frequency of the elements other than oxygen in II and

[0099] p and q are numbers whose ratio p/q is from 0.1 to 10,

[0100] containing three-dimensional regions of the chemical compositionY¹ _(a′)Y² _(b′)O_(x′) which are delimited from their local environmentowing to their composition differing from their local environment andwhose maximum diameter (longest distance passing through the center ofgravity of the region and connecting two points present on the surface(interface) of the region) is from 1 nm to 100 μm, frequently from 10 nmto 500 nm or from 1 μm to 50 or 25 μm.

[0101] Particularly advantageous novel mixed oxide active materials IIare those in which Y¹ is bismuth.

[0102] Among these in turn those which correspond to the formula III

[Bi_(a″)Z² _(b″)O_(x″)]_(p″)[Z² ₁₂Z³ _(c″)Z⁴ _(d″)Fe_(e″)Z⁵ _(f″)Z⁶_(g″)Z⁷ _(h″)O_(y″)]_(q)   (III),

[0103] where

[0104] Z² is molybdenum and/or tungsten,

[0105] Z³ is nickel and/or cobalt,

[0106] Z⁴ is thallium, an alkali metal and/or an alkaline earth metal,

[0107] Z⁵ is phosphorus, arsenic, boron, antimony, tin, cerium and/orlead,

[0108] Z⁶ is silicon, aluminum, titanium and/or zirconium,

[0109] Z⁷ is copper, silver and/or gold,

[0110] a″ is from 0.1 to 1,

[0111] b″ is from 0.2 to 2,

[0112] c″ is from 3 to 10,

[0113] d″ is from 0.02 to 2,

[0114] e″ is from 0.01 to 5, preferably from 0.1 to 3,

[0115] f″ is from 0 to 5,

[0116] g″ is from 0 to 10,

[0117] h″ is from 0 to 1,

[0118] x″ and y″ are each numbers which are determined by the valencyand frequency of the elements other than oxygen in III,

[0119] p″ and q″ are each numbers whose ratio p″/q″ is from 0..1 to 5,preferably from 0.5 to 2,

[0120] are preferred, very particularly preferred materials III beingthose in which Z² _(b″) is (tungsten)b and Z² ₁₂ is (molybdenum)₁₂.

[0121] It is also advantageous if at least 25 mol % (preferably at least50, particularly preferably at least 100, mol %) of the total moiety [y¹_(a′)y² _(b′)O_(x′)]_(p) ([Bi_(a″)Z² _(b″)O_(x″)]_(p″)) of the mixedoxide active materials II (mixed oxide active materials III) are presentin these mixed oxide active materials in the form of three-dimensionalregions of the chemical composition y¹ _(a′)y² _(b′)O_(x′) (Bi_(a″)Z²_(b″)O_(x″)) which are delimited from the local environment owing totheir chemical composition differing from their local environment andwhose largest diameter is from 1 nm to 100 μm.

[0122] Regarding the shaping, the statements made in the case of themixed oxide active materials I are applicable with regard to the mixedoxide active materials II.

[0123] A large number of the mixed oxide active materials suitable forthe heterogeneously catalyzed gas-phase partial oxidation of acrolein toacrylic acid can be subsumed under the formula IV

Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)   (IV),

[0124] where

[0125] X¹ is W, Nb, Ta, Cr and/or Ce,

[0126] X² is Cu, Ni, Co, Fe, Mn and/or Zn,

[0127] X³ is Sb and/or Bi,

[0128] X⁴ is one or more alkali metals,

[0129] X⁵ is one or more alkaline earth metals,

[0130] X⁶ is Si, Al, Ti and/or Zr,

[0131] a is from 1 to 6,

[0132] b is from 0.2 to 4,

[0133] c is from 0.5 to 18,

[0134] d is from 0 to 40,

[0135] e is from 0 to 2,

[0136] f is from 0 to 4,

[0137] g is from 0 to 40 and

[0138] n is a number which is determined by the valency and frequency ofthe elements other than oxygen in IV.

[0139] Preferred embodiments within the mixed oxide active materials IVare those which are described by the following meanings of the variablesin the formula IV:

[0140] X¹ is W, Nb, and/or Cr,

[0141] X² is Cu, Ni, Co, and/or Fe,

[0142] X³ is Sb,

[0143] X⁴ is Na and/or K,

[0144] X⁵ is Ca, Sr and/or Ba,

[0145] X⁶ is Si, Al, and/or Ti,

[0146] a is from 1.5 to 5,

[0147] b is from 0.5 to 2,

[0148] c is from 0.5 to 3,

[0149] d is from 0 to 2,

[0150] e is from 0 to 0.2,

[0151] f is from 0 to 1 and

[0152] n is a number which is deteremined by the valency and frequencyof the elements other than oxygen in IV.

[0153] However, very particularly preferred mixed oxide active materialsIV are those of the formula V

Mo₁₂V_(a′)Y¹ _(b′)Y² _(c′)Y⁵ _(f′)Y⁶ _(g′)O_(n′)  (V)

[0154] where

[0155] Y¹ is W and/or Nb,

[0156] Y² is Cu and/or Ni,

[0157] Y⁵ is Ca and/or Sr,

[0158] Y⁶ is Si and/or Al,

[0159] a′ is from 2 to 4,

[0160] b′ is from 1 to 1.5,

[0161] c′ is from 1 to 3,

[0162] f′ is from 0 to 0.5,

[0163] g′ is from 0 to 8 and

[0164] n′ is a number which is determined by the valency and frequencyof the elements other than oxygen in V.

[0165] The mixed oxide active materials (IV) suitable according to theinvention are obtainable in a manner known per se, for example disclosedin DE-A 4335973 or in EP-A 714700.

[0166] In principle, mixed oxide active materials suitable according tothe invention and of the formula IV can be prepared in a simple mannerby producing, from suitable sources of their elemental constituents, avery intimate, preferably finely divided dry blend having a compositioncorresponding to their stoichiometry and calcining said dry blend atfrom 350 to 600° C. The calcination can be carried out either underinert gas or under an oxidizing atmosphere, e.g. air (mixture of inertgas and oxygen) or under a reducing atmosphere (e.g. a mixture of inertgas and reducing gases such as H₂, NH₃, CO, methane and/or acrolein orsaid reducing gases by themselves). The duration of calcination may befrom a few minutes to a few hours and usually decreases with increasingtemperature. Suitable sources of the elemental constituents of the mixedoxide active materials IV are those compounds which are already oxidesand/or those compounds which can be converted into oxides by heating, atleast in the presence of oxygen.

[0167] The thorough mixing of the starting compounds for the preparationof mixed oxide active materials IV can be effected in dry or in wetform. If it is effected in dry form, the starting compounds areexpediently used in the form of finely divided powder and, after mixingand, if required, compaction, are subjected to calcination. However,thorough mixing is preferably effected in wet form.

[0168] Usually, the starting compounds are mixed with one another in theform of an aqueous solution and/or suspension. Particularly intimate dryblends are obtained in the mixing process described when the startingmaterials used are exclusively sources of the elemental constituentspresent in dissolved form. A preferably used solvent is water. Theaqueous material obtained is then dried, the drying process preferablybeing carried out by spray-drying of the aqueous mixture at outlettemperatures from 100 to 150° C.

[0169] The mixed oxide active materials IV suitable according to theinvention can be used for the novel process, for example, after beingshaped to give annular catalyst geometries, it being possible to effectthe shaping before or after the final calcination, in a manner fullycorresponding to that in the case of the mixed oxide active materials I.For example, annular unsupported catalysts can be prepared completelyanalogously from the powder form of the mixed oxide active material orits uncalcined precursor material by compaction to give the desiredcatalyst geometry (for example by extrusion) where, if required,assistants, e.g. graphite or stearic acid as lubricants and/or moldingassistants and reinforcing agents, such as microfibers of glass,asbestos, silicon carbide or potassium titanate, may be added.

[0170] The shaping of the pulverulent active material or itspulverulent, still uncalcined precursor material may of course also beeffected by application to inert catalyst support preshaped into annularform. The coating of the supports for the production of the coatedcatalysts is carried out as a rule in a suitable rotatable container, asdisclosed, for example, in DE-A 2909671, EP-A 293859 or EP-A 714700.

[0171] For coating the supports, the powder material to be applied isexpediently moistened and, after the application, is dried, for exampleby means of hot air. The coat thickness of the powder material appliedto the support is expediently chosen to be in the range from 10 to 1000μm, preferably from 50 to 500 μm, particularly preferably from 150 to250 μm.

[0172] The support materials used may be conventional porous ornonporous aluminas, silica, thorium dioxide, zirconium dioxide, siliconcarbide or silicates, such as magnesium silicate or aluminum silicate.The supports having pronounced surface roughness are preferred. Thefineness of the catalytically active oxide materials to be applied tothe surface of the support is of course adapted to the desired coatthickness (cf. EP-A 714 700).

[0173] According to the invention, the mixed oxide active materials IVcan of course also be shaped to give annular supported catalysts.

[0174] Advantageous mixed oxide active materials to be used according tothe invention for the gas-phase partial oxidation of acrolein to acrylicacid are furthermore materials of the formula VI

[D]_(p)[E]_(q)   (VI),

[0175] where

[0176] D is Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶_(g″)O_(x″),

[0177] E is Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″),

[0178] Z¹ is W, Nb, Ta, Cr and/or Ce,

[0179] Z² is Cu, Ni, Co, Fe, Mn and/or Zn,

[0180] Z³ is Sb and/or Bi,

[0181] Z⁴ is Li, Na, K, Rb, Cs and/or H

[0182] Z⁵ is Mg, Ca, Sr and/or Ba,

[0183] Z⁶ is Si, Al, Ti and/or Zr,

[0184] Z⁷ is Mo, W, V, Nb and/or Ta,

[0185] a″ is from 1 to 8,

[0186] b″ is from 0.2 to 5,

[0187] c″ is from 0 to 23,

[0188] d″ is from 0 to 50,

[0189] e″ is from 0 to 2,

[0190] f″ is from 0 to 5,

[0191] g″ is from 0 to 50,

[0192] h″ is from 4 to 30,

[0193] i″ is from 0 to 20 and

[0194] x″ and y″ are numbers which are determined by the valency andfrequency of the elements other than oxygen in VI and

[0195] p and q are numbers which differ from zero and whose ratio p/q isfrom 160:1 to 1:1,

[0196] which are obtainable by separately preforming a multimetal oxidematerial E

Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″)  (E),

[0197] in finely divided form (starting material 1) and thenincorporating the preformed solid starting material 1 into an aqueoussolution, an aqueous suspension or a finely divided dry blend of sourcesof the elements Mo, V, Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶, which contain theabovementioned elements in the stoichiometry D

Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶ _(g″)  (D),

[0198] (starting material 2), in the desired ratio p:q, drying anyresulting aqueous mixture and calcining the resulting dry precursormaterial at from 250 to 600° C. before or after it is shaped to give thedesired catalyst geometry.

[0199] Mixed oxide active materials VI where the preformed solidstarting material 1 is incorporated into an aqueous starting material 2at <70° C. are preferred. A detailed description of the preparation ofactive material comprising mixed oxide VI appears in, for example, EP-A668104, DE-A 19736105 and DE-A 19528646.

[0200] Regarding the shaping, the statements made in the case of theactive materials comprising mixed oxide IV are applicable with regard tothe active materials comprising mixed oxide VI.

[0201] Mixed oxide active materials particularly suitable for theheterogeneously catalyzed gas-phase partial oxidation of methacrolein tomethacrylic acid are disclosed, for example, in DE-A 19 815 279 and inthe prior art cited in this publication.

[0202] Mixed oxide active materials particularly suitable for theheterogeneously catalyzed gas-phase partial oxidation of propane toacrylic acid are disclosed, for example, in DE-A 10 051 419 and in theprior art cited in this publication.

[0203] As in all other cases, the shaping required for the novel processcan be effected in the case of the abovementioned mixed oxide activematerials too by coating suitable supports or, for example, by extrusionprocesses.

[0204] The novel process is preferably carried out in tube-bundlereactors loaded with the catalyst, as described, for example, in EP-A700 714 and EP-A 700 893 and in the literature cited in thesepublications.

[0205] In the abovementioned tube-bundle reactors, the catalyst tubesare usually produced from ferritic steel and typically have a wallthickness of from 1 to 3 mm. The internal diameter is as a rule from 20to 30 mm, frequently from 21 to 26 mm. In terms of applicationtechnology, the number of catalyst tubes housed in the tube-bundlecontainer is at least 5000, preferably at least 10000. Frequently, thenumber of catalyst tubes housed in the reaction container is from 15000to 30000. Tube-bundle reactors having more than 40000 catalyst tubestend to be the exception. Inside the container, the catalyst tubes areusually homogeneously distributed, the distribution expediently beingchosen so that the distance between the central inner axes of adjacentcatalyst tubes (i.e. the catalyst tube spacing) is from 35 to 45 mm (cf.for example EP-B 468 290).

[0206] Particularly suitable heat-exchange media are fluid heatingmedia. The use of melts of salts, such as potassium nitrate, potassiumnitrite, sodium nitrite and/or sodium nitrate, or of low-melting metals,such as sodium, mercury and alloys of different metals, is particularlyadvantageous.

[0207] Detailed information on the reaction conditions expediently to bemaintained for the various novel heterogeneously catalyzed gas-phasepartial oxidations of precursor compounds of (meth)acrylic acid is to befound in the prior art cited above.

[0208] The novel procedure is particularly suitable for carrying outheterogeneously catalyzed gas-phase partial oxidations of precursorcompounds of (meth)acrylic acid which are carried out with high loadingof the fixed catalyst bed with the precursor compound, as effected, forexample, in DE-A 19 948 523.

[0209] Such high-load gas-phase partial oxidations are preferablyrealized in the multi-zone (preferably two-zone) tube-bundle reactors ofDE-A 19 948 523.

[0210] The advantage of the novel procedure is primarily an increasedselectivity of the formation of the desired products, in particular withthe use of high loadings of the fixed catalyst bed with the precursorcompound of (meth)acrylic acid.

[0211] The size of the catalyst bodies used according to the inventionis as a rule such that the longest dimension (longest line connectingtwo points present on the surface of the catalyst support) is from 2 to12 mm, frequently from 4 to 8 mm.

EXAMPLES

[0212] A) Preparation of a Multimetal Oxide Active Material Suitable forthe Heterogeneously Catalyzed Gas-Phase Partial Oxidation of Propene toAcrolein and Shaped into Rings of Various Dimensions

[0213] At 60° C., 213 kg of ammonium heptamolybdate were dissolved inportions in 600 l of water. 0.97 kg of a 46.8% strength by weightaqueous potassium hydroxide solution at 20° C. was stirred into thissolution while maintaining the 60° C. (a solution A was obtained). Asecond solution B was prepared by adding 116.25 kg of an aqueous ironnitrate solution (14.2% by weight of Fe) to 333.7 kg of an aqueouscobalt nitrate solution (12.4% by weight of Co) at 30° C. whilestirring. After the end of the addition, stirring was carried out for afurther 30 minutes at 30° C. 112.3 kg of an aqueous bismuth nitratesolution (11.2% by weight of Bi) were then stirred in at 60° C. to givethe solution B. The solution B was stirred into the solution A at 60° C.in the course of 30 minutes. 15 minutes after the end of the stirringin, 19.16 kg of silica sol (46.80% by weight of SiO₂, density: 1.36 to1.42 g/ml, pH from 8.5 to 9.5, alkali content not more than 0.5% byweight) were added to the resulting slurry at 60° C. Stirring wascarried out for a further 15 minutes while maintaining the 60° C. Theslurry obtained was then spray-dried by the countercurrent method (gasinlet temperature: 400±10° C., gas outlet temperature: 140±5° C.), aspray-dried powder whose loss on ignition (3 hours at 600° C. under air)was 30% of its weight being obtained.

[0214] In each case 1.5% by weight of finely divided graphite (sieveanalysis: min. 50% by weight <24 μm, max. 10% by weight >24 μm and <48μm, max. 5% by weight >48 μm, BET surface area: from 6 to 13 m²/g) wereadditionally mixed into portions of the resulting spray-dried powder(particle size of the spray-dried powder was about 30 μm).

[0215] The dry blend resulting in each case was compacted (compressed)to give hollow cylinders (rings) of different geometries so that theresulting density was about 2.5 mg/mm³ and the resulting lateralcompressive strength of the rings was about 10 N.

[0216] For the final calcination, in each case 1900 g of the shapedrings were poured into a heatable through-circulation chamber (0.12 m³internal volume, 2 m³ (S.T.P.) of air/minute). The temperature in thebed was then changed as follows:

[0217] increased from 25° C. to 160° C. at 1° C./min.;

[0218] then kept at 160° C. for 100 min.;

[0219] then increased from 160° C. to 200° C. at 3° C./min;

[0220] then kept at 200° C. for 100 min.;

[0221] then increased from 200° C. to 230° C. at 2° C./min.;

[0222] then kept at 230° C. for 100 min.;

[0223] then increased from 230° C. to 270° C. at 3° C./min.;

[0224] then kept at 270° C. for 100 min.;

[0225] then increased to 380° C. at 1° C./min.;

[0226] then kept at 380° C. for 4.5 h;

[0227] then increased to 430° C. at 1° C./min.;

[0228] then kept at 430° C. for 4.5 h;

[0229] then increased to 500° C. at 1° C./min.;

[0230] then kept at 500° C. for 9 h;

[0231] then cooled to 25° C. in the course of 4 h.

[0232] Annular catalyst bodies were obtained.

[0233] B) Heterogeneously Catalyzed Gas-Phase Partial Oxidation ofPropene to Acrolein

[0234] A reaction tube (V2A stainless steel, 30 mm external diameter; 2mm wall thickness; 26 mm internal diameter, length: 439 cm) was loadedfrom bottom to top, on a catalyst support ledge (44 cm long), first withsteatite beads having a rough surface (from 4 to 5 mm diameter; inertmaterial for heating the reaction gas starting mixture) over a length of30 cm and then in each case with the catalyst rings produced under A)over a length of 270 cm (the fixed catalyst bed), before the loading wascompleted with the abovementioned steatite beads as a downstream bedover a length of 30 cm. The remaining catalyst tube length was leftempty.

[0235] That part of the reaction tube which had been loaded with solidwas thermostatted by means of 11 cylindrical aluminum blocks which hadbeen cast around the tube, each had a length of 30 cm and was heated byelectric heating tapes (comparative experiments using a correspondingreaction tube heated by means of a salt bath through which nitrogen wasbubbled showed that thermostatting with the aluminum block was capableof simulating thermostatting with a salt bath). Those ends of thereaction tube which were free of solid were kept at 220° C. with steamunder superatmospheric pressure.

[0236] The reaction tube described above was continuously fed with areaction gas starting mixture having the following composition:

[0237] 6.5% by volume of propene,

[0238] 3.5% by volume of H₂O,

[0239] 0.5% by volume of CO,

[0240] 1.2% by volume of CO₂,

[0241] 0.04% by volume of acrolein and

[0242] 10.7% by volume of O₂,

[0243] the remaining amount to 100% by volume comprising molecularnitrogen.

[0244] The loading of the fixed catalyst bed was chosen as 100 l (S.T.P)of propene/1·h. A small sample of the product gas mixture was taken atthe exit for a gas chromatographic analysis. The temperature of allaluminum blocks was set at a standard value in all cases so that thepropene conversion in all cases was 95 mol % in a single pass. Thetemperatures required for this purpose were about 330° C.

[0245] The table below shows the selectivity of the acrolein formation(S_(A)) achieved as a function of the catalyst geometry used. The letterC indicates that it is a comparative example, while the letter E showsthat it is an example according to the invention.

[0246] Other meanings are:

[0247] Ex=external diameter,

[0248] H=height and

[0249] In=internal diameter

[0250] of the catalyst ring.

[0251] In addition, the table shows, as example E5, the result when acatalyst body having the geometry according to FIG. 1 is used.

[0252] The preparation was carried out as in the case of the catalystrings, except that the precursor material was compressed to give theother geometry.

[0253] The diameter and the height of the base body were 4 mm.

[0254] The groove depth was about 0.5 mm. The spacing of the groovesegment ends was about 0.9 mm. TABLE Ex (mm) H (mm) In (mm) V_(B) (cm³)V_(BA) (cm³) A_(B) (cm²) A_(B)/V_(B) (cm¹⁻) V_(B)/V_(BA) S_(A) (mol %)C1 5 3 2 0.049 0.059 0.990 20 0.84 86.4 C2 5 3 2.5 0.044 0.059 1.00122.7 0.75 87.9 C3 5 3 3 0.038 0.059 1.005 26.7 0.64 89.1 C4 5 2 2 0.0330.039 0.770 23.3 0.84 85.3 C5 5 2 2.5 0.029 0.039 0.766 26.0 0.75 88 C65 2 3 0.025 0.039 0.754 30.0 0.64 89.1 C7 5.5 3 2.5 0.057 0.071 1.13120.0 0.79 88.3 C8 5.5 3 3 0.050 0.071 1.135 22.7 0.70 90.1 E1 5.5 3 3.50.042 0.071 1.131 26.7 0.59 91.2 C9 6 3 3 0.064 0.085 1.272 20.0 0.7589.6 E2 6 3 4 0.047 0.085 1.257 26.7 0.56 92.2 C10 7 3 3 0.094 0.1151.571 16.7 0.82 87.6 C11 7 3 4 0.078 0.115 1.555 20.0 0.67 89.8 E3 7 34.5 0.068 0.115 1.535 22.7 0.59 91.9 E4 7 3 5 0.057 0.115 1.508 26.70.49 92.4 E5 — — — 0.031 0.050 0.828 27.0 0.61 92.5

We claim:
 1. A process for the heterogeneously catalyzed gas-phasepartial oxidation of a precursor compound of (meth)acrylic acid to(meth)acrolein and/or (meth)acrylic acid by passing a reaction gasstarting mixture comprising the precursor compound, molecular oxygenand, if required, a gas which is inert with respect to the catalyticgas-phase partial oxidation, at elevated temperatures, through a fixedcatalyst bed which contains, as the catalyst, a mixed oxide activematerial shaped into a geometric body, this geometric body being ageometric base body into whose surface at least one cavity has beenintroduced, wherein the ratio of the volume of the geometric body V_(B)to the volume of the geometric base body V_(BA)≦0.63 and the ratio ofthe external surface area of the geometric body A_(B) to V_(B)≧22 cm⁻¹.2. A process as claimed in claim 1, wherein A_(B):V_(B)≧24 cm⁻¹.
 3. Aprocess as claimed in claim 1 or claim 2, wherein V_(B):V_(BA)≦0.60. 4.A process as claimed in any of claims 1 to 3, wherein the geometric basebody is selected from the set comprising cylinders, pyramids, cones,cubes, right parallelepeds, prisms, spheres, truncated cones andtruncated pyramids.
 5. A process as claimed in any of claims 1 to 4,wherein the geometric body is a ring.
 6. A process as claimed in claim5, wherein the ring geometry (external diameter×height×internaldiameter) is selected from: 5.5 mm×3 mm×3.5 mm;   a) 6 mm×3 mm×4 mm;  b) 7 mm×3 mm×4.5 mm and   c) 7 mm×3 mm×5 mm.   d)
 7. A process asclaimed in any of claims 1 to 6, wherein the empty volume of the fixedcatalyst bed is ≧50% by volume.
 8. A process as claimed in any of claims1 to 7, wherein the mixed oxide active material is one which contains a)the elements Mo, Cu and P or b) the elements Mo, Bi and Fe or c) theelements Mo, V and W or d) the elements Mo, V, Te and Nb.
 9. A processas claimed in any of claims 1 to 8, wherein, in a single pass of thereaction gas mixture through the fixed catalyst bed, the conversion ofthe precursor compound of (meth)acrylic acid ≧90 mol %.
 10. A process asclaimed in claim 9, wherein the selectivity of the formation of(meth)acrolein and/or (meth)acrylic acid ≧50 mol %.
 11. A process asclaimed in any of claims 1 to 10, which is a process for theheterogeneously catalyzed gas-phase partial oxidation of propene toacrolein and wherein the mixed oxide active material is one of theformula I Mo₁₂Bi_(a)Fe_(b)X¹ _(c)X² _(d)X³ _(e)X⁴ _(f)O_(n)   (I), whereX¹ is nickel and/or cobalt, X² is thallium, an alkali metall and/or analkaline earth metal, X³ is zinc, phosphorus, arsenic, boron, antimony,tin, cerium, lead and/or tungsten, X⁴ is silicon, aluminum, titaniumand/or zirconium, a is from 0.5 to 5, b is from 0.01 to 5, c is from 0to 10, d is from 0 to 2, e is from 0 to 8, f is from 0 to 10 and n is anumber which is determined by the valency and frequency of the elementsother than oxygen in I.
 12. A process as claimed in any of claims 1 to10, which is a process for the heterogeneously catalyzed gas-phasepartial oxidation of acrolein to acrylic acid and wherein the mixedoxide active material is one of the formula IV Mo₁₂V_(a)X¹ _(b)X² _(c)X³_(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)   (IV), where X¹ is W, Nb, Ta, Cr and/orCe, X² is Cu, Ni, Co, Fe, Mn and/or Zn, X³ is Sb and/or Bi, X⁴ is one ormore alkali metals, X⁵ is one or more alkaline earth metals, X⁶ is Si,Al, Ti and/or Zr, a is from 1 to 6, b is from 0.2 to 4, c is from 0.5 to18, d is from 0 to 40, e is from 0 to 2, f is from 0 to 4, g is from 0to 40 and n is a number which is determined by the valency and frequencyof the elements other than oxygen in IV.
 13. A process as claimed in anyof claims 1 to 12, which is carried out at from 200 to 450° C.
 14. Amixed oxide active material shaped into a geometric body, wherein thisgeometric body is a geometric base body into whose surface at least onecavity has been introduced, wherein the ratio of the volume of thegeometric body V_(B) to the geometric base body V_(BA)≦0.63 and theratio of the external surface area of the geometric body A_(B) toV_(B)≧22 cm⁻¹ and wherein the mixed oxide active material is one whichcontains a) the elements Mo, Cu and P or b) the elements Mo, Bi and Feor c) the elements Mo, V and W or d) the elements Mo, V, Te and Nb. 15.The use of a (meth)acrylic acid prepared by a process as claimed inclaim 1 for the preparation of esters of (meth)acrylic acid or for thepreparation of polymers.